The present invention relates generally to micromachining and microfabrication technology, and more particularly to the construction of integrated micro-miniature structures based on a departure from conventional thin-film polysilicon fabrication techniques.
Micro-technology encompasses a number of integrated microfabrication techniques derived from processes applicable to semiconductors and integrated circuits. These techniques include chemical etching, reactive ion etching, oxide etching, dry plasma etching, metallization, metal deposition, photolithography, thermal diffusion, ion implantation, and chemical vapor deposition. In recent years micromachining and microfabrication techniques have been used to advantage to provide a variety of active and passive microstructures in large quantities and at low cost. Among the many structures proposed are actuators and sensors, micro-motors and a variety of other devices including movable joints, levers, gears, sliders, springs and the like.
Historically, most of the surface-machined microstructures have been polysilicon-based. These structures are fabricated conventionally by chemical-vapor-deposited polycrystalline silicon films that are etched to produce structural layers on the order of 1-2 .mu.m thick. These minimal thicknesses represent an inherent limitation of polysilicon-based structures which reduces their effectiveness in some applications. For example, polysilicon microactuators have been produced that are capable of generating constant forces on the order of sub-micro Newtons and torques on the order of sub-nano Newton-Meters. These devices can be used to move small objects at Mhz frequencies. On the other hand, it would be desirable to move objects having a size of one cubic millimeter or more at acceleration rates as high as 100G. For that purpose, forces on the order of milli-Newtons will be required. The fabrication of microstructures at this level requires a design providing sufficient vertical or out-of-plane rigidity, yet very low mechanical stiffness in the direction of acceleration such that the energy available for moving an object is maximized.
U.S. Pat. No. 5,025,346 proposes the construction of laterally driven resonant microactuator structures using polysilicon thin-film techniques in a sacrificial system. Fabrication is based on a four mask process wherein a 2 .mu.m thick layer of polysilicon is deposited by low pressure chemical vapor deposition (LPCVD) over a sacrificial layer of phosphosilicate glass (PSG). Following appropriate patterning, the sacrificial layer is dissolved to yield a free standing actuator structure and suspension system. The suspension system includes a plurality of polysilicon beams that support the actuator for high frequency lateral movement in the plane of the structure. At a vertical thickness only 2 .mu.m, however, very small vertical stiffness, that is, stiffness normal to the desired direction of acceleration, is provided by the suspension beams. This low vertical stiffness causes large vertical movements which are not desirable. Fortunately, as thickness in a given direction increases, the mechanical stiffness in that direction increases as a function of the cube of the thickness. On the other hand, stiffness in the orthogonal direction increases only linearly.
Thus, in microstructure applications, particularly those having free standing structures, it would be desirable to increase vertical structural thickness. This will require fabrication methods that overcome the thickness limitations of conventional thin film deposition techniques. Moreover, due to constraints on structural thickness in the direction of acceleration, techniques enabling the fabrication of structures having high aspect ratios (i.e., the ratio of vertical to lateral thickness) will be required. Although various x-ray techniques have been proposed for generating structures having thicknesses on the order of 100 .mu.m, these thicknesses are much larger than necessary. Rather, it would be desirable to provide microstructures having vertical thicknesses on the order of 10 .mu.m at an aspect ratio of about 10:1.